Displays in image display devices, such as television devices, are now being shifted from conventional cathode-ray tube displays to thin displays, such as liquid crystal displays and plasma displays. With the thin displays, the thicknesses of the image display devices can be reduced. Liquid crystal panels included in the liquid crystal display devices do not emit light, and thus backlight devices are required as separate lighting devices. The backlight devices are classified broadly into a direct type and an edge-light type according to mechanisms thereof. For realizing further reduction in thicknesses of the liquid crystal display devices, the edge-light type backlight devices including light guide plates are preferably used, and one described in PTL 1 below is known as one example thereof.